Digital Protection Control System and Digital Protection Control Apparatus

ABSTRACT

Disclosed are a digital protection control system and a digital protection control apparatus, wherein the digital protection control apparatus can easily be made to have more terminals, even when the number of terminals of a power transmission line increases. The digital protection control system has, as terminal stations thereof, a reference station that is to become the reference point for the sampling time at which power grid current information is to be taken in, tail-end stations that take in power grid current information from the power grid system, and intermediate stations that are connected between the reference station and the tail-end stations via transmission paths. The intermediate station is provided with an uplink transmission unit that is connected to a transmission path at the reference station side thereof, and a plurality of downlink transmission units that are connected to transmission paths at the tail-end station side thereof. The intermediate station also has the time thereof to be subordinate to the time of the terminal station connected to the uplink transmission unit thereof, and the time of the terminal stations connected to the downlink transmission units thereof are made to be subordinate to the intermediate station, with the time thereof used as reference time.

TECHNICAL FIELD

The present invention relates to a digital protection control system anddigital protection control apparatus which detect a failure andabnormality occurring in a power system.

BACKGROUND ART

Against a failure occurring in a power system so-called short circuitfailure or earth faulting, a protection relay device installed in eachelement such as a transmission line, transformer, bus, and generatorforming a power system operates with respect to a failure occurringwithin its territory. When a section containing the failure point isseparated from the left trouble-free power system through tripping of abreaker, a failure is removed. At present, there goes mainstream adigital protection relay in which current information or voltageinformation of power system is digitized and algorithm for determining asystem failure is realized through software process.

As its function, hopefully, a power protection relay correctly detects afailure within a previously allocated protection object and fastseparates from a power system a section which a failure occurrencesection can be minimized. Accordingly, ideally, a device referred to asa main protection device which ought to operate at first at the timewhen a failure occurs fast operates if at all possible in an internalfailure and fails to operate in an external failure. Accordingly, as amethod for accurately detecting a failure, a current differentialprinciple is widely used for a relay for the main protection device.

However, as in a transmission line protection device, currentinformation of both ends of transmission lines is necessary for using acurrent differential principle. In this case, a relay is installed inboth ends of the transmission line and both the ends are connected by acommunication line. At least at one end, the current information unitsof both the ends are collected, thereby using the current differentialprinciple. Through congestion of transmission lines, multiple-terminaltransmission lines are currently used and transmission lines having fiveor six terminals are more used.

As can be seen from the above sequence, current information in whichsampling synchronization of all terminals is accomplished andcommunication paths for collecting the current information are necessaryfor applying a protection relay of the current differential principle toa multiple-terminal transmission line. As a method for configuring acommunication path in the case where a multiple-terminal transmissionline is used, there are used a representative terminal judgment type inwhich from a representative terminal, the left terminals are connectedthrough communication lines in a radial pattern and data of allterminals is collected to the representative terminal, and a loop systemin which a relay is installed in each terminal, a terminal is connectedto each other in a loop form through a communication line, and data ofall terminals is collected (see, for example, NON PATENT LITERATURE 1).

For realizing a protection of a high-accuracy transmission line,high-precision sampling synchronization is needed. A samplingsynchronization method in which a delay in a transmission path isconsidered is each applied to a configuration of the communication path(see, for example, NON PATENT LITERATURE 1).

CITATION LIST Non Patent Literature

NON PATENT LITERATURE 1: “Protective relay system engineering” edited byYoshifumi Ooura, Institute of Electrical Engineers, pp. 158-159, 164-165

SUMMARY OF INVENTION Technical Problem

Recently, a transmission line tends to be increasingly provided withmultiple terminals due to the facts that acquisition of site for atransforming station is difficult and a power supply is dispersed.Terminals are expanded with respect to existing transmission lines inmany cases. Along with the above, a transmission line protection devicealso needs to respond to expansion of terminals. Also, a communicationpath, as a matter of course, needs to be changed.

In the aforementioned representative terminal judgment type ofconventional technology, a communication path is relatively simple.However, a representative terminal needs to have communication ports asmuch as the number of terminals to be used, and physical communicationports need to be increased according to the increase in the number ofterminals. Along with the above, relay calculation processes also needto be increased based on the receiving process of communication or thereceived data, and the above is a restriction during the increase in thenumber of the terminals.

On the other hand, in a system using communication paths of conventionalloop system, the number of communication ports of each terminal fails toincrease. However, for collecting data of all terminals, data issequentially transferred in a loop form between terminals. That is, dataneeds to be relayed and the number of times of transferring dataincreases according to the increase in the number of the terminals. Thisis an important factor in obstructing a high speed operation as a relaydevice, and also is a restriction during the increase in the number ofthe terminals.

As described above, when multiple-terminal transmission lines areconventionally used, there is also a restriction on the side of thecurrent differential relay device which protects transmission lines, andit is a factor in obstructing a multiple-terminal operation.

In view of the foregoing, it is an object of the present invention toprovide a digital protection control system and digital protectioncontrol apparatus capable of easily implementing a multiple-terminalcurrent differential relay system.

Solution to Problem

To accomplish the above objects, according to one aspect of the presentinvention, there is provided a digital protection control system. Thisdigital protection control system includes as a terminal station: areference station configured to take system current information andfunction as a reference of sampling time; a tail-end station configuredto take system current information from a power system; and anintermediate station configured to be connected through a transmissionpath between the reference station and the tail-end station, wherein:the digital protection control system transmits and receives systemcurrent information between a plurality of the terminal stations andperforms protection control of the power system; the intermediatestation includes an upper transmission unit connected to a transmissionpath of the reference station side and a plurality of lower transmissionunits connected to a transmission path of the tail-end station side; andsubordinates time of the intermediate station to time as a reference ofa terminal station connected to the upper transmission unit, andsubordinates time of a terminal station connected to the lowertransmission unit to time as a reference of the intermediate station.

Advantageous Effects of Invention

Through a configuration according to the present invention, a protectioncontrol system for a multiple-terminal transmission line capable ofeasily expanding terminals can be constructed while securing a samplingsynchronization from a power system. Accordingly, for example, aterminal can be easily added due to dispersion of power supply, or aterminal of the transmission line can be easily added due to generationof a new load.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a configuration example of a relay terminal devicewhich constitutes a relay system according to the present embodiment;

FIG. 2 illustrates a configuration example of a relay system having alower station of three terminals;

FIG. 3 illustrates a configuration example of eight terminals in which acommunication path is configured by using a stage configuration of fourstages;

FIG. 4 illustrates a configuration example of a terminal dedicated to alowermost station;

FIG. 5 illustrates a configuration example of a terminal dedicated to anuppermost station;

FIG. 6 illustrates a configuration example of a calculation terminaldedicated to an uppermost station;

FIG. 7 illustrates a configuration example of a calculation terminaldedicated to an intermediate station;

FIG. 8 illustrates an example in which a communication path isconfigured by using a stage configuration of three stages;

FIG. 9 illustrates a configuration example of a system having acommunication path of a stage configuration of three stagescorresponding to a protection of a nine-terminal transmission line;

FIG. 10 illustrates an example of a characteristic diagram of a currentdifferential relay;

FIG. 11 illustrates an example of a section protection relay accordingto an application of the present embodiment; and

FIG. 12 illustrates an example of a protection relay corresponding to along-distance line according to an application of the presentembodiment.

DESCRIPTION OF EMBODIMENTS

Preferred embodiments of the present invention will now be described indetail below with reference to the accompanying drawings.

First Embodiment

FIG. 1 illustrates a configuration example of a relay terminal devicewhich constitutes a relay system according to the present embodiment. Aterminal station 1 being a terminal device of a protection relaygenerates a sampling reference signal 1-11SP based on a clock of its ownstation by using a reference clock unit, shown as “C.L.” in the drawing,1-07. An uplink 1-03 being a transmission unit for an uplink stationincludes a transmission interface unit 1-04 and a reception interfaceunit 1-05. In the case where a terminal station connected to a sitebeyond the transmission interface unit 1-04 and the reception interfaceunit 1-05 is located, the uplink 1-03 is provided on a clock controlcircuit, shown as “C.C.C.” in the drawing, 1-06 for synchronizing itsown station with clock timing of the upper station.

In the case where this communication partner is present, the clockcontrol circuit 1-06 gives to the reference clock unit 1-07 a timingcontrol, shown as “T.C.” in the drawing, signal 1-06 a for correcting aclock being a signal for synchronizing a clock of its own station withan upper station connected to an uplink. A calculation unit, shown as“C.U.” in the drawing, 1-01 is connected to the uplink 1-03 via a systembus 1-02.

In the relay system, an input circuit 1-13 is used for taking ON/OFFinformation such as external device information. An output circuit 1-14is used for breaker control or auxiliary relay control during a relayoperation. An analog filter unit 1-12 which attenuates a specificfrequency domain, a sample-and-hold circuit 1-11 which samples and takesan analog input signal, and an analog-digital converter unit 1-10 whichperforms analog-digital conversion to a sampled signal each have acircuit for sampling and digitizing current or voltage information ofthe system side. Sampling timing of this circuit is determined by asampling reference signal 1-11 SP generated by the reference clock unit1-07.

In the present embodiment, a downlink 1-15 being a transmission unit fora downlink station is further provided, and a circuit for subordinatingit to a clock of the reference clock unit 1-07 of this terminal stationis provided in this communication partner destination. In thiscommunication control circuit, as contrasted with the above-describeduplink 1-03, there is used a circuit operating as a master clock withrespect to the communication partner destination. The downlink 1-15reads out a synchronization reference signal 1-07R for synchronizing thedownlink station side with its own station from the reference clock unit1-07. The downlink 1-15 further transmits it to the communicationpartner destination via the transmission interface unit 1-15-1 for thedownlink 1-15 and subordinates the communication partner destination tothis signal. In addition, the downlink 1-15 receives current informationof a power system from the communication partner destination via thereception interface unit 1-15-2 for the downlink 1-15.

In the present embodiment, two or more downlinks are further provided.In the present embodiment, descriptions will be described as aconfiguration in which three downlinks are totally provided, and thereis a meaning in a configuration in which two or more downlinks areprovided. Here, a downlink 1-16 is set as a second downlink, and adownlink 1-17 is set as a third downlink. The downlinks 1-16 and 1-17are also synchronized with the reference clock unit 1-07, and there isno difference from the downlink 1-15 in a point for subordinating thecommunication partner destination to the synchronization referencesignal. A transmission interface unit 1-16-1 for the downlink 1-16 and atransmission interface unit 1-17-1 for the downlink 1-17 each have thesame configuration as that of the transmission interface unit 1-15-1.Further, a reception interface unit 1-16-2 for the downlink 1-16 and areception interface unit 1-17-2 for the downlink 1-17 each have the sameconfiguration as that of the reception interface unit 1-15-2.

The present embodiment has a hybrid configuration capable of having afunction of a master station as a clock master, subordinating at leasttwo terminal stations to the master station through a downlink, andbeing subordinated to one terminal station through an uplink.

When adopting this configuration, the terminal station 1 is used as astarting point as illustrated in FIG. 2 and terminal stations areconnected in a radial pattern from the downlink by using a terminalstation having the exact same configuration, thereby expanding thesystem in a tree form. A terminal station 1A which subordinates its ownstation to a clock of the terminal station 1 being an upper stationconnects an uplink side of its own station to the downlink side of theterminal station 1, thereby making the subordinate synchronization.Terminal stations 1B and 1C are the same as the terminal station 1A, andthe terminal station 1 functions as a master clock of the four terminalstations.

A configuration in which terminal stations are further increased isillustrated in FIG. 3. In this example, when the terminal station 1A isfurther configured as an intermediate station by using the downlink ofthe terminal station 1A, three terminal stations such as terminalstations 1AA, 1AB, and 1AC are expanded.

In this example, an example in which a terminal station 1AAAsubordinated to the terminal station 1AA is connected thereto is furtherillustrated. In this case, the terminal station 1AAA is synchronizedwith a clock of the terminal station 1AA, the terminal stations 1AA,1AB, and 1AC are synchronized with the terminal station 1A, and theterminal stations 1A, 1B, and 1C are synchronized with the terminalstation 1, thereby securing a sampling synchronization of the entiresystem. When the terminal station 1 is further connected to a downlinkof a new terminal station from the uplink side of its own station, it isfurther synchronized with an upper clock. When a new terminal station isconnected to the downlink of the terminal stations 1B, 1C, 1AA, 1AB,1AC, and 1AAA, a terminal station can be expanded while securing asampling synchronization of the system.

In this configuration, by using the system configuration illustrated inFIG. 1, an example in which terminal stations of an uppermost station toa tail-end station are constructed is illustrated and a downlink isunnecessary for the tail-end terminal station. On the other hand, anuplink is unnecessary for the uppermost terminal station. Accordingly,there may be used a configuration dedicated to the tail-end terminalstation in which a function of the downlink is removed from the terminalstation configuration of FIG. 1 as illustrated in FIG. 4, or aconfiguration dedicated to the uppermost terminal station in which afunction of the uplink is conversely removed from the terminal stationconfiguration of FIG. 1 as illustrated in FIG. 5. Further, there may beused a configuration in which an analog input unit is removed from theuppermost terminal station and in which the calculation unit, the inputcircuit, and the output circuit are left.

FIG. 7 illustrates a configuration dedicated to the intermediateterminal station, and a configuration in which the uplink and thedownlink do not differ from those of FIG. 1 and the analog input unit iseliminated. FIG. 8 illustrates an example in which the terminal stationillustrated in FIG. 6 is arranged in the uppermost terminal station 6,terminal stations dedicated to the intermediate terminal stationillustrated in FIG. 7 are arranged in the intermediate terminal stations7A, 7B, and 7C, and a terminal station dedicated to the tail-endapparatus illustrated in FIG. 4 is arranged in the lowermost terminalstations 4A, 4B, and 4C arranged in a lower terminal station of theintermediate terminal station 7A.

From the intermediate terminal stations 7B and 7C, communication ports7B-1, 7B-2, 7B-3, 7C-1, 7C-2, and 7C-3 further connected to lowerterminal stations will be omitted subsequently; further, a terminalstation having a configuration of the intermediate terminal stations maybe connected or a terminal station dedicated to a tail-end terminalstation may be connected thereto.

As can be seen from the above sequence, the terminal station accordingto the present embodiment has a clock as a reference of the samplingsynchronization, and has at least two or more downlinks being acommunication port for a lower terminal station of a terminal forsubordinating its own station to this clock. The terminal stationfurther has one uplink being a communication port for an upper terminalstation for subordinating this clock to a clock of a communicationpartner destination. In transmission information between respectivecommunication ports, the terminal station transmits information on thetime or clock between terminal stations to each other, and performs timesetting or clock setting so that the lower station can perform datasampling to the upper station at the same time.

Further, the system having a configuration using this terminal stationnecessarily connects the downlink to the uplink of a communicationpartner destination except for the tail-end terminal station as thelowermost terminal station. On the other hand, the uplink is connectedto the downlink of the communication partner destination except for theuppermost terminal station. As can be seen from the above sequence, whenconfiguring a system in which terminal stations having treelike topologyare connected, a sampling synchronization of the entire system is set soas to be synchronized with the uppermost terminal station of the tree.The system can be configured based on the sampling synchronization dataof the entire system and a terminal station can be easily expanded.

Next, a method for implementing a current differential protection relaysystem for suitable multiple-terminal transmission lines by usingterminal stations of the system configuration according to the presentembodiment will be further described.

The current differential protection relay system need to cyclicallyperform a current differential calculation using current data of allterminals at the same time. However, as the number of terminals moreincreases, the amount of data to be treated more increases by a multiplenumber of the number of terminals. Therefore, relay calculationprocessing needs to similarly be increased based on reception processingof communication or received data, and this is a restriction during theincrease in the number of the terminals.

In a transmission path configuration using a conventional loop system,for example, as the number of the terminals more increases, the amountof data passing through a transmission path more increases as much asthe number of the terminals, and therefore it becomes difficult to addthe number of the terminals beyond a limitation of capacity oftransmission. Also in the conventional representative terminal judgmenttype, it is similarly obvious that as the number of the terminals moreincreases, the reception processing or calculation processing of eachterminal more increases. Accordingly, it becomes difficult to add thenumber of the terminals beyond a limitation of the reception processingor calculation processing.

In the current differential protection relay system using the terminalstation according to the present embodiment, when the system isconstructed, for example, as illustrated in FIG. 9, even if the numberof the terminals increases, the system can be constructed withoutincreasing a communication capacity of transmission.

In FIG. 9, there is illustrated an example in which a transmission pathamong terminals is constructed in treelike topology by using a9-terminal transmission line and the relay terminal station of thepresent embodiment. Through the system configuration of the presentembodiment, the sampling synchronization of all the terminal stations isperformed with respect to a master terminal station 9-1-7 located in thetop of the tree. Accordingly, there is secured the samplingsynchronization of nine terminal stations 9-1-3, 9-2-3, 9-3-3, 9-4-3,9-5-3, 9-6-3, 9-7-3, 9-8-3, and 9-9-3 located in the tail end oftransmission lines.

Here, when all the data units are simply transferred to the side of theupper terminal stations, the data to be treated on the upper terminalstation side increases according to multiplication. For the purpose oftransferring data to the side of the upper terminal station, thetransmission capacity needs to be increased in the order correspondingto the upper terminal station side.

To solve the above-described problem, the present embodiment has aconfiguration in which without simply transferring information receivedfrom the lower terminal stations to the upper terminal stations, theintermediate terminal station calculates only a suppression amountnecessary for a vectorial sum of current and a relay calculation byusing current data having secured therein the sampling synchronizationreceived from the lower terminal station side, and transmits onlycalculation results to the upper terminal station side. The presentembodiment further has a configuration in which the upper terminalstation further calculates additional processing and a suppressionamount with respect to a vector sum of the current received from aplurality of terminal stations on the lower side and further transmitsthem to an upper terminal station. Finally, the present embodiment has aconfiguration in which an uppermost terminal station receives the vectorsum of current and scalar sum of the suppression amount calculated byeach intermediate terminal station, and calculates a size ofdifferential current and a scalar sum from the vector sum of the entiresampling current based on this data, thereby performing the relaycalculation.

Through this configuration, even if the number of the terminalsincreases and a plurality of transmission paths are constructed, themultiple-terminal current differential relay can be implemented withoutincreasing the upper transmission capacity. Through this configuration,there can be implemented a system configuration in which a transmissionamount to the upper terminal station side is one amount of currentvector sum and one amount of suppression amount with respect to oneelement of relay as it is and does not depend on the number of theterminals.

Next, a specific relay calculation method will be described withreference to a 9-terminal transmission line. A calculation formula ofthe current differential relay in the 9-terminal transmission line is asfollows. When the current information units of each terminal are set asI₁ to I₉, a differential current operation element Id is defined as asize of the vector sum of each current (formula 10-001).

As an actual calculation method, an effective value calculation isperformed with respect to results of adding all instantaneous values ofcurrent sampled at the same time in each tail-end terminal station.Examples of the calculation method of effective values include a methodsuch as an arithmetic mean of a square value and an arithmeticapproximation of an absolute value. The suppression amount to bring arate to a relay characteristic is further defined as a scalar sum ofeach current (formula 10-002).

A size of each current is calculated by an effective value calculation,thereby implementing this calculation method. Formulae obtained byestablishing the formulae 10-001 and 10-002 to nine terminal portionsare represented as formulae 10-001a and 10-002a.

Id=|ΣI _(i)|  (formula 10-001)

Id=|I ₁ +I ₂ +I ₃ +I ₄ +I ₅ +I ₆ +I ₇ +I ₈ +I ₉|  (formula 10-001a)

I _(R) =Σ|Ii|  (formula 10-002)

I _(R) =|I ₁ |+|I ₂ |+|I ₃ |+|I ₄ |+|I ₅ |+|I ₆ |+|I ₇ |+|I ₈ |+|I₉|  (formula 10-002a)

According to a size of the current, the relay characteristic having arate in a relay sensitivity is given by using minimum operation currentK2 and a rate K1 by

Id≧K1·I _(R)   (formula 10-003)

Id≧K2  (formula 10-003A).

In FIG. 10, there is illustrated an example of a characteristic diagramof the current differential relay illustrating a relationship betweenthe differential current operation element Id and the suppression amountI_(R). Ordinarily, a vector sum of instantaneous values of current in aterminal of each transmission line is synthesized and the differentialcurrent operation element Id is calculated by a size of its vector. Asthe suppression amount I_(R), a value obtained by simply adding sizes ofcurrent in a terminal of each transmission line is used. The suppressionamount I_(R) is suppressed so as not to increase the differentialcurrent operation element.

Actually, according to a size of current, a relay characteristic havinga plurality of rate characteristics of intermediate current range andlarge current range is further known; however, it fails to harm thepresent embodiment. Hereinafter, a calculation method of the presentembodiment for implementing the current differential relay formultiple-terminal transmission lines will be described.

In the intermediate terminal station, a calculation of vector sum andscalar sum of the suppression amount is supposed to be performed withrespect to current information transmitted from the lower terminalstation. Further, only the calculation results are supposed to betransmitted to the upper terminal station to compress information.

The current vector sum and scalar sum in the intermediate terminalstation are defined as follows.

Id _(INT) =I ₁ +I ₂ +I ₃  (formula 10-004)

I _(RINT) =|I ₁ |+|I ₂ |+|I ₃|  (formula 10-005)

Instantaneous values of current data acquired at the same time in eachtail-end terminal station are added to obtain the formula 10-004. Theformula 10-005 represents a scalar sum of the current data acquired ineach tail-end terminal station. This calculation method does not differfrom the formula 10-002 at all.

Suppose that an intermediate terminal station 1 acquires terminalinformation of terminals 1 to 3, an intermediate terminal station 2acquires terminal information of terminals 4 to 6, and an intermediateterminal station 3 acquires terminal information of terminals 7 to 9.When the differential current vector sum and scalar sum calculated ineach of the intermediate terminal stations are defined as Id_(INT1),I_(RINT1), Id_(INT2), I_(RINT2), Id_(INT3), and I_(RINT3), respectively,

Id _(INT1) =I ₁ +I ₂ +I ₃  (formula 10-006),

I _(RINT1) =|I ₁ |+|I ₂ |+|I ₃|  (formula 10-007),

Id _(INT2) =I ₄ +I ₅ +I ₆  (formula 10-008),

I _(RINT2) =|I ₄ |+|I ₅ |+|I ₆|  (formula 10-009),

Id _(INT3) =I ₇ +I ₈ +I ₉  (formula 10-010),

and

I _(RINT3) =|I ₇ |+|I ₈ |+|I ₉|  (formula 10-0011).

Based on the information from three intermediate terminal stations, theupper terminal station receiving the calculation results calculates thedifferential current vector sum and scalar sum through the sameprocedure. When the results are set as Id_(MST) and I_(RMST),

Id _(MST) =Id _(INT1) +Id _(INT2) +Id _(INT3)  (formula 10-012)

and

I _(RMST) =I _(RINT1) +I _(RINT2) +I _(RINT3)  (formula 10-013).

From the above, when the formulae are developed, there are obtained

Id _(MST) =I ₁ +I ₂ +I ₃ +I ₄ +I ₅ +I ₆ +I ₇ +I ₈ +I ₉  (formula 10-014)

and

I _(RMST) =|I ₁ |+|I ₂ |+|I ₃ |+|I ₄ |+|I ₅ |+|I ₆ |+|I ₇ |+|I ₈ |+|I₉|  (formula 10-015).

When an effective value is further calculated with respect to the resultof the formula 10-014, the relay characteristic illustrated in FIG. 10is matched with a feasible electric charge.

Also in the case where a transmission configuration has three or morestages, the above fact is not changed. The intermediate terminal stationalways calculates only the sum of current vector and the sum of thescalar amount through a part of current information performed in thelower terminal station, and further transfers calculation results to theupper terminal station. The uppermost terminal station calculates thecurrent vector sum and the scalar sum and then calculates the effectivevalue with respect to the calculation results of the current vector sum,thereby implementing the multiple-terminal current differential relaycalculation.

When the results of this relay calculation is traced through a conversecommunication route, and transferred from the uppermost terminal stationto the intermediate terminal station and further from the intermediateterminal station to the tail-end terminal station, thereby notifying theentire system of the calculation results of the current differentialrelay. In the same manner as in the current differential relay system ofthe existing representative terminal judgment type, the tail-endterminal station performs tripping control of a breaker according to thecalculation results of the current differential relay.

As described above, the intermediate terminal station does not increasea transmission capacity of the upper terminal station side bycalculating a current vector sum and a scalar sum. The uppermostterminal station can calculate a current differential relay andimplement a multiple-terminal current differential relay. Further, thetransmission amount of the upper terminal station side is one amount ofthe current vector sum and one amount of the suppression amount. Even ifthe number of terminals increases and the number of stages of atransmission path increases, this is not changed. The present embodimenthas a configuration in which a multiple-terminal current differentialrelay system capable of flexible expansion can be implemented againstthe increase of the number of terminals.

In the present embodiment, there is solved a problem in multipleterminals of a current differential relay of transmission lines.Further, the present embodiment provides a current differential relaysystem capable of having multiple terminals in which even if the numberof terminals increases, a communication amount for transmission is notincreased, even if the number of terminals increases, special hardwareneed not be developed, and a processing burden for software is notincreased.

Second Embodiment

In the present embodiment, eve if a terminal station iscommunication-connected to multiple stages, sampling synchronization ofall terminals is secured. An example in which a section protectionsystem applicable to a multiple-section transmission line is constructedas its application is illustrated in FIG. 11.

Section electric power stations 11-A, 11-B, 11-C, 11-D, and 11-Econstituting a transmission network are electric power stations ofswitchyards building a transmission network, and may be transformerstations having switching equipment. In the present embodiment, theelectric power stations 11-A to 11-E are constituted through onetransmission line. Here, 11-S01 is set as a transmission line section 1,11-S02 is set as a transmission line section 2, 11-S03 is set as atransmission line section 3, and 11-S04 is set as a transmission linesection 4. Current information units S01A and S01B are those of bothends of the section 1, and taken from a sensor or CT. In the samemanner, current information units S02A and S02B are those of both endsof the section 2, current information units S03A and S03B are those ofboth ends of the section 3, and current information units S04A and S04Bare those of both ends of the section 4.

In electric power stations, terminal stations according to the presentembodiment are arranged, respectively. Suppose that the terminal station11-001 is arranged in the electric power station 11-A, the terminalstation 11-002 is arranged in the electric power station 11-B, theterminal station 11-003 is arranged in the electric power station 11-C,the terminal station 11-004 is arranged in the electric power station11-D, and the terminal station 11-005 is arranged in the electric powerstation 11-E. Suppose further that current information units of bothends of the transmission line are taken from a sensor or CT provided oneach electric power station.

In the present embodiment, a configuration in which the terminal station11-001 is communication-connected as a top of a tree will be described.The terminal station 11-001 is first connected sequentially, and atail-end station is the terminal station 11-005 in the presentembodiment. Through the transmission configuration, a samplingsynchronization in which the terminal station 11-001 is implemented as areference clock is secured.

Suppose in the present embodiment that a current differential relayfunction of each power transmission section is implemented on the sidein which a clock having a high priority is present among terminalstations installed in both ends of a transmission line. In the presentembodiment, the current differential relay function of the transmissionline section 11-S01 is implemented on the terminal station 11-001. Inthe same manner, the current differential relay function of thetransmission line section 11-S02 is implemented on the terminal station11-002, the current differential relay function of the transmission linesection 11-S03 is implemented on the terminal station 11-003, and thecurrent differential relay function of the transmission line section11-S04 is implemented on the terminal station 11-004.

Through this configuration, the current information to be transmitted tothe upper terminal station through the uplink from each terminal stationmay be only information for one terminal station portion. For example,the terminal station 11-005 takes the current information S04B andtransmits it as the current information I-S04B to the terminal station11-004 through the uplink. The terminal station 11-004 can perform acurrent differential relay calculation by using the current informationS04A sampled in its own terminal station and the received currentinformation I-S04B. The terminal station 11-004 transmits a relaycalculation result RY-S04 to the terminal station 11-005 through thedownlink, thereby performing tripping control at both ends during therelay operation.

From the above, the current differential relay system of the section 4can be constructed. By using the same procedure, the currentdifferential relay system can be constructed also in the other sectionsand, for example, the terminal station 11-004 transmits the currentinformation I-S03B to the terminal station 11-003. The terminal station11-003 performs the current differential relay calculation by using thecurrent information S03A sampled in its own terminal station and thereceived current information I-S03B. When the terminal station 11-003transmits the relay calculation result RY-S03 to the terminal station11-004, the current differential relay system of the section 3 can beconstructed. Much the same is true on the other sections.

Conventionally, in the case where a current differential relay system isapplied to a transmission line having a plurality of sections, a currentdifferential relay device independent from the other sections needs tobe installed at both ends of transmission lines in each section.According to the terminal station of the present embodiment, a relayterminal to be installed may be one set in each electric power station,and an inexpensive current differential relay system can be constructed.

The present embodiment further has a configuration in which in the casewhere any terminal station is not connected to the uplink side, itfunctions as a reference clock. Also in case communication is disruptedand a failure of a terminal station occurs, a function as a currentdifferential relay is kept between trouble-free terminal stations.Accordingly, the present embodiment provides a relay system in which allfunctions fail to receive effects and which has high reliability.

In the present embodiment, a simple transmission line of one line withtwo terminals is illustrated. The present embodiment expands terminalsin a tree form, and through the process, can easily correspond to atransmission line having a plurality of lines or a configuration ofmultiple terminals.

Third Embodiment

As described also in the present embodiment, in principle, a protectionrelay device for a transmission line current differential needs currentinformation on a partner terminal station in which samplingsynchronization is secured and has a communication function. However, anoptical interface for communication composed of laser diodes andphotodiodes has a limitation to a transmission distance without a relaydevice because of the attenuation of optical signals due to opticalfibers.

Therefore, conventionally, a signal is repeated through a specialcommunication device, thus implementing a long distance correspondence.Communication devices are constructed by signal multiplexing devices anddeveloped as a device for telephone or data transmission. However,through the progress of IP technology, the communication devices arerequired to be suitable for, for example, a PDH network to a high-speedpacket communication, than a conventional cyclic communication network.For this purpose, there includes a big problem in future a combinationof communication apparatus composed of a protection relay system whichneeds a sampling synchronization performance of microsecond order and apacket communication network in which a delay time relies on traffic anda communication time is probabilistically changed.

In the present embodiment, there will be described a system in whichwhen using the terminal station of the present embodiment, a samplingsynchronization is easily secured even if a communication path isconfigured by a terminal station in multiple stages, and when a relay isperformed by the same terminal station, a communication distance can beeasily extended.

This configuration is illustrated in FIG. 12. Terminal stations 12-01and 12-02 are the terminal stations for relay according to the presentembodiment illustrated in FIG. 4. Further, terminal stations 12-03 and12-04 are the terminal stations according to the present embodimentillustrated in FIG. 7. In this configuration, the terminal station 12-01functions as a reference clock of the entire system. According to thisclock, the terminal stations 12-03, 12-04, and 12-02 establish thesampling synchronization.

In the present embodiment, since the terminal stations 12-03 and 12-04each function as a simple relay device, the terminal station 12-02receives from the downlink side system information on the sampledvoltage or current from the system side and sequentially transmits it tothe uplink side. Finally, the information sampled by the terminalstation 12-02 is transferred to the terminal station 12-01, thusperforming a relay calculation. The relay calculation results aresequentially transmitted from the downlink of the terminal station 12-01to the lower terminal station side. Further, they are finallytransferred to the terminal station 12-02, thus performing the controlas the protection relay. As described above, the present embodiment hasa terminal configuration in which a relay for transmission can be easilyimplemented while securing the sampling synchronization.

As described above, the present embodiment can standardize terminalstations configuring a multiple-terminal current differential relaysystem. Through the process, a terminal station having the same systemconfiguration is combined in multiple stages even if the number ofterminals increases, thereby expanding the number of terminals of thecurrent differential relay.

Even if a terminal station having a hardware configuration matched witha shape of transmission lines is not developed, a current differentialrelay system of the representative terminal judgment type can be easilyconstructed by using the terminal station of the present embodiment.

A system using the terminal stations according to the present embodimenthas a configuration in which by using current data in which the samplingsynchronization received from the lower terminal station side issecured, only a vector sum of current and a suppression amount necessaryfor a final relay calculation are calculated and only the calculationresults are transmitted to the upper terminal station side. The systemfurther has a configuration in which in the upper terminal station side,additional processing and the suppression amount are further calculatedto the vector sum of current received from a plurality of lower terminalstation side, and calculation results are further transmitted to theupper terminal station. Through the above-described configuration, evenin the case where the number of terminals increases and a systemconfiguration has multiple stages, a current differential relay havingmultiple terminals can be implemented without increasing a transmissioncapacity of the upper terminal station side.

In the above-described embodiments, a current differential relay whichcommunicates current information is described, and the terminal stationaccording to the present embodiment can collect sampling synchronizationdata of wide area. The terminal stations can be applied to applicationsof protection relay system, stabilizing device, and fault point locationdevice necessary for sampling information on voltage or current of powersystem for different points. Further, the terminal station can beapplied not only to the sampling information on current or voltage butalso to a wide-area protection system in which device information andcalculation results of relays are collected.

REFERENCE SIGNS LIST

-   1, 1A˜1C, 1AA, 1AB, 1AC, 1AAA, 9-1-3˜9-9-3, 11-001˜11-005,    12-001˜12-004 Terminal station-   1-01, 4-01, 5-01, 6-01, 7-01 Calculation unit-   1-02, 4-02, 5-02, 6-02, 7-02 System bus-   1-03, 4-03, 7-03 Uplink-   1-04, 1-15-1, 1-16-1, 1-17-1, 4-04, 5-15-1, 5-16-1, 5-17-1, 6-15-1,    6-16-1, 6-17-1, 7-04, 7-15-1, 7-16-1, 7-17-1 Transmission interface    unit-   1-05, 1-15-2, 1-16-2, 1-17-2, 4-05, 5-15-2, 5-16-2, 5-17-2, 6-15-2,    6-16-2, 6-17-2, 7-05, 7-15-2, 7-16-2, 7-17-2 Reception interface    unit-   1-06, 4-06, 7-06 Clock control circuit-   1-06 a, 4-06 a, 7-06 a Timing control signal-   1-07, 4-07, 7-07 Reference clock unit-   1-07R, 5-07R, 6-07R, 7-07R Synchronization reference signal-   1-1, 4-11, 5-11 Sample-and-hold circuit-   1-10 Analog-digital converter unit-   1-11SP, 4-11SP, 5-11SP Sampling reference signal-   1-12, 4-12, 5-12 Analog filter unit-   1-13, 4-13, 5-13, 6-13, 7-13 Input circuit-   1-14, 4-14, 5-14, 6-14, 7-14 Output circuit-   1-15, 1-16, 1-17, 5-15, 5-16, 5-17, 6-15, 6-16, 6-17, 7-15, 7-16,    7-17 Downlink-   4-10, 5-10 Analog-digital converter-   4A˜4C Lowermost terminal station-   5-07, 6-07 Reference clock unit as a master of all terminal stations-   6 Uppermost terminal station-   6L-1 Communication route connected to a lower intermediate terminal    station 7A from the uppermost terminal station 6-   6L-2 Communication route connected to an intermediate terminal    station 7B from the uppermost terminal station 6-   6L-3 Communication route connected to an intermediate terminal    station 7C from the uppermost terminal station 6-   7A, 7B, 7C, 9-1-4˜9-1-6 Intermediate terminal station-   7AL-1 Communication route connected to the lowermost terminal    station 4A from the intermediate terminal station 7A-   7AL-2 Communication route connected to the lowermost terminal    station 4B from the intermediate terminal station 7A-   7AL-3 Communication route connected to the lowermost terminal    station 4C from the intermediate terminal station 7A-   7B-1, 7B-2, 7B-3 Communication port further connected to a lower    terminal station from the intermediate terminal station 7B-   7C-1, 7C-2, 7C-3 Communication port further connected to a lower    terminal station from the intermediate terminal station 7C-   9-1, 9-2, 9-3, 9-4, 9-5, 9-6, 9-7, 9-8, 9-9 Transforming station in    a tail end of transmission line 9-1L-   9-1L Transmission line of 9 terminals-   9-1-1 CT for measuring current in a tail end of transmission line of    the terminal 9-1-   9-1-2 CT secondary cable connected to a relay 9-1-3 from the 9-1-1    CT-   9-1-7 Master terminal station-   9-1-3U, 9-2-3U, 9-3-3U, 9-4-3U, 9-5-3U, 9-6-3U, 9-7-3U, 9-8-3U,    9-9-3U Transmission path for connecting a lower terminal station and    an intermediate terminal station-   9-1-4U, 9-1-5U, 9-1-6U Communication path for connecting an    intermediate terminal station and the uppermost intermediate    terminal station 9-1-7-   11-A, 11-B, 11-C, 11-D, 11-E Electric power station-   11-S01 Transmission line section 1-   11-S02 Transmission line section 2-   11-S03 Transmission line section 3-   11-S04 Transmission line section 4-   Id Differential current operation element-   I_(R) Suppression amount-   S01A˜S04A, S01B˜S04B, I-S01B˜I-S04B Current information-   RY-S01˜RY-S04 Relay calculation result

1. A digital protection control system comprising as a terminal station:a reference station configured to take system current information andfunction as a reference of sampling time; a tail-end station configuredto take system current information from a power system; and anintermediate station configured to be connected through a transmissionpath between the reference station and the tail-end station, wherein:the digital protection control system transmits and receives systemcurrent information between a plurality of the terminal stations andperforms protection control of the power system; and the intermediatestation includes an upper transmission unit connected to a transmissionpath of the reference station side and a plurality of lower transmissionunits connected to a transmission path of the tail-end station side, andsubordinates time of the intermediate station to time as a reference ofa terminal station connected to the upper transmission unit, andsubordinates time of a terminal station connected to the lowertransmission unit to time as a reference of the intermediate station. 2.The digital protection control system according to claim 1, wherein theintermediate station calculates a scalar sum and vector sum of thesystem current information transmitted from a terminal station connectedto the lower transmission unit, and transmits a calculation result to aterminal station connected to the upper transmission unit.
 3. Thedigital protection control system according to claim 2, wherein theintermediate station takes system current information from a powersystem based on time of the intermediate station, calculates a vectorsum and scalar sum matched with the taken system current information,and transmits a calculation result to a terminal station connected tothe upper transmission unit.
 4. The digital protection control systemaccording to claim 2, wherein the reference station or the intermediatestation performs a protection control calculation of a power systembased on a vector sum and scalar sum of the system current informationtransmitted from the terminal station connected to the reference stationor the intermediate station.
 5. The digital protection control systemaccording to claim 1, wherein: the intermediate station performs a relaycalculation for determining abnormality of a power system by usingsystem current information to be received from the terminal stationconnected to the lower transmission unit, and transmits a calculationresult to the terminal station connected to the lower transmission unit;and the terminal station selects and transmits system currentinformation necessary for a relay calculation to the terminal stationconnected to the upper transmission unit.
 6. A digital protectioncontrol system comprising: a plurality of terminal stations configuredto be connected through a transmission path, wherein: the digitalprotection control system transmits and receives system currentinformation of a power system between the plurality of terminalstations, and performs protection control of the power system; theplurality of terminal stations: include an uplink transmission unit towhich the terminal station is connected as a reference of time of theterminal station, and a plurality of downlink transmission units towhich the terminal station subordinating time of the terminal station toa reference is connected; in the case where there is a terminal stationconnected to the uplink transmission unit, synchronize time of theterminal station based on the synchronizing signal transmitted from theterminal station connected to the uplink transmission unit; and in thecase where there is a terminal station connected to the downlinktransmission unit, transmit a synchronizing signal for synchronizingtime of the terminal station to a reference to a terminal stationconnected to the downlink transmission unit.
 7. A digital protectioncontrol system comprising: a reference station configured to take systemcurrent and have a clock functioning as a reference of sampling time; aplurality of intermediate stations configured to be connected to thereference station through a transmission path and synchronize a clock ofthe intermediate station with a clock of the reference station; and aplurality of tail-end stations configured to be connected to oneintermediate station among the plurality of intermediate stationsthrough a transmission path, and synchronize a clock of the tail-endstation with a clock of the intermediate station, and take systemcurrent information from a power system based on the synchronized clockof the tail-end station.
 8. The digital protection control systemaccording to claim 7, wherein: the plurality of tail-end stationstransmit the system current information taken from a power system to theintermediate station connected to the tail-end station; and theintermediate station calculates differential current and suppressionamount of a plurality of system current information units transmittedfrom the plurality of tail-end stations, and transmits a calculationresult to the reference station.
 9. The digital protection controlsystem according to claim 8, wherein the intermediate station takessystem current information from a power system based on a clock of theintermediate station, and calculates differential current andsuppression amount matched with the taken system current information.10. A digital protection control apparatus comprising: an uplinktransmission unit configured to transmit system current information to areference terminal as a reference of time, and receive a clock referencesignal from the reference terminal; a clock unit configured tosynchronize time of the clock unit with a clock reference signal inputthrough the uplink transmission unit, and output a clock subordinatesignal indicating time of the clock unit; and a plurality of downlinktransmission units configured to transmit, in the case where a terminalis connected, the clock subordinate signal to the terminal and receivesystem current information from the terminal.
 11. The digital protectioncontrol apparatus according to claim 10, further comprising, in the casewhere the terminal is connected to at least two downlink transmissionunits among the plurality of downlink transmission units, a calculationunit configured to calculate a vector sum and scalar sum of a pluralityof system current information units received through at least the twodownlink transmission units, wherein the vector sum and the scalar sumare transmitted to the reference terminal via the uplink transmissionunit.
 12. The digital protection control apparatus according to claim11, further comprising a sampling unit configured to take system currentinformation from a power system based on a sampling reference signaloutput from the clock unit, wherein the calculation unit calculates avector sum and scalar sum of a plurality of system current informationunits received via the downlink transmission unit and the system currentinformation taken by the sampling unit, and transmits the vector sum andthe scalar sum to the reference terminal via the uplink transmissionunit.
 13. A digital protection control system to be connected inmultiple stages among a plurality of electric power stations oftransmission network, perform a relay calculation for determiningabnormality in multiple sections of a power system by a plurality ofcontrol terminals in which synchronization of sampling time of systemcurrent is accomplished, and protect a power system, comprising: when amaster terminal as a reference of the sampling time is set as an upperterminal among the plurality of control terminals, the plurality ofcontrol terminals except the master terminal perform samplingsynchronization processing of the control terminal based on time of thecontrol terminal connected to an upper side; select and transmit systemcurrent information necessary for a relay calculation performed by thecontrol terminal of the upper side with respect to the control terminalof the upper side; and receive a calculation result of the relaycalculation from the control terminal of the upper side.